In hydraulic power systems the viscosity of the hydraulic fluid changes as the fluid temperature changes. Changes in the viscosity of the fluid affect the pressure drop as the fluid flows through the lines and other elements of the circuit, the change of pressure drop being usually proportional to the change in fluid viscosity. Furthermore, pump efficiency and the resulting pump output flow are also viscosity sensitive and also affect the pressure drop. Under unusually cold ambient temperature conditions, the viscosity of the hydraulic fluid increases considerably which can reduce performance of the system and possibly cause a malfunction. This is particularly true since some hydraulic elements such as control lines or pump inlet lines are designed to operate within a narrow pressure drop range.
The adverse effects of a change of viscosity of a hydraulic fluid in a control system have been known for years. Furthermore, attempts in the past have been made to correct for or cancel the effect of a change of viscosity including the use of a viscosity sensitive elongated resistance or capillary. However, the past teachings normally utilize complicated hydraulic circuits including pressure regulator valves in order to maintain the desired pressure and flow.
One such teaching is U.S. Pat. No. 2,005,731 wherein a viscosity sensitive resistance is in series with a variable restriction. The change in pressure drop across the resistance, caused by an increase in viscosity of the fluid, is utilized to control a pressure regulator valve which modulates the flow of the control fluid to drain from a point in a hydraulic circuit between the viscosity sensitive resistance and the variable restriction. It is particularly noted that both the sensing means and the control means are in series with the working cylinder. Thus an increase in viscosity of the fluid causes a pressure drop through both the viscosity sensitive resistance and a pressure drop through the variable restriction which are in series with each other.
U.S. Pat. No. 3,922,853 discloses a hydraulic circuit with a viscosity sensitive capillary in parallel with a first adjustable restriction and in series with a second adjustable restriction. However, a pressure regulator valve is used to control the output pressure. Thus, the output pressure is not established by either restriction, but by the pressure regulator valve.
U.S. Pat. No. 4,167,853 teaches a hydrostatic vehicle transmission control which has a capillary or throttle in series with a fixed orifice to compensate for a change in viscosity of the control fluid. However, also located in the circuit is a spring biased pressure relief valve which limits the pressure, not the flow, of the system. The viscosity sensitive capillary thus does not modify an adverse viscosity induced flow characteristic across a flow control means.